Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already declare readOnly, idempotent, non-destructive, and open-world, so the safety profile is covered. The description adds genuinely new behavioral context: a hard free quota of 10 calls/day with a per-call price and two accepted payment paths (x402 via params._meta or HTTP, X-API-Key credits), which affects whether and how an agent should retry or batch.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

The functional sentence is front-loaded and dense with the information an agent needs first. The second sentence is billing boilerplate but each element (free quota, price, payment channels) is decision-relevant; the bracketed server attribution is the only clearly low-value fragment.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

The tool is simple and an output schema exists, so return-field explanation is unnecessary, and the description still previews the outputs (patch, counts, similarity). Combined with rate-limit and payment disclosure and full schema coverage, an agent has enough to invoke it correctly; only explicit sibling routing is absent.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 100%, so the schema already documents a (old text), b (new text), and mode (lines | words | chars). The description repeats the mode vocabulary as "line, word or character" but adds no extra meaning such as default mode, case sensitivity, or whitespace handling.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

"Line, word or character diff between two texts with a unified patch, added/removed counts and similarity" gives a specific verb (diff), the resource (two texts), the modes, and the outputs. The phrase "two texts" implicitly separates it from csv_diff/json_diff siblings, but no sibling is named, so differentiation is inferred rather than stated.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

There is no explicit when-to-use guidance: nothing tells the agent to prefer text_similarity for a single score, csv_diff for tabular data, or json_diff for structured payloads. The only operational guidance is commercial (free tier, payment mechanics), not task-selection guidance.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.